ABSTRACT Alzheimer's disease and related dementias (AD/RD) are pervasive and debilitating diseases currently affecting over 5 million Americans. At age 65, the lifetime risk of developing dementia is 21.1% among women compared to 11.6% among men. Although age is the most influential risk factor for dementia, the greater life expectancy of women compared to men does not entirely explain the higher prevalence of dementia among women. Given the higher burden of AD/RD, it is particularly important to identify risk factors for AD/RD among women. In addition, a growing body of research underscores the need to examine midlife risk factors for dementia, especially since dementia pathology often begins decades before diagnosis. Our overarching goal is to leverage data from the existing Women's Health Study (WHS) to explore a large range of risk factors for AD/RD among women followed from middle age into old age. The goal of this proposal is to expand the infrastructure of the WHS to allow us to assess AD/RD in a cohort that was not originally designed to evaluate these important and timely questions. The WHS began in 1992 as a randomized trial testing aspirin, beta- carotene, and vitamin E for the primary prevention of cancer and cardiovascular disease in 39,876 apparently healthy women aged ?45 years. The women have been followed observationally since the end of the trial in 2004. As of the end of December 2018, average mean follow-up was almost 24 years, with morbidity follow-up still close to 90% and mortality follow-up virtually 100%. An extensive body of data is available, with decades of yearly follow-up questionnaires on a wide range of demographic, lifestyle, and medical history variables. Since the mean age of the participants is currently 77.1 years (range 67.3 to 106.3 years), social security numbers are available on virtually all participants, and Centers for Medicare & Medicaid Services (CMS) data are available even for participants unwilling or unable to return study questionnaires, we propose to obtain and assess data from CMS to identify dementia diagnoses among WHS participants from 2011 to 2018. There is no other feasible and cost-effective strategy to assess dementia in a geographically diverse cohort the size of the WHS. We will also assess the association between established demographic, lifestyle, medical history, and genetic risk factors for dementia using previously collected data from WHS questionnaires and dementia data from CMS, and compare our findings to the current literature. These aims are feasible to accomplish within the one-year time frame of this administrative supplement and will provide us with important information on the feasibility of using CMS data to obtain information on AD/RD diagnoses in the WHS. In the future, we will apply for funding to explore novel risk factors for dementia among women, leveraging from the WHS the broad range of risk factor data collected over 24 years, the large sample size of women, and the availability of genetic data. The WHS has made substantial contributions to our understanding of cancer and cardiovascular disease, and we believe that the WHS can be effectively leveraged to understand AD/RD risk factors among women.